The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device presenting black display when the liquid crystal layer thereof takes on a roughly vertically aligned state.
In recent years, liquid crystal display devices have come into wide use thanks to their improvement in display quality. However, further improvement in display quality is yet strongly desired.
One of the display properties of liquid crystal display devices of which further improvement is demanded is reduction in viewing angle dependence of display quality. That is, it is desired to develop a liquid crystal display device capable of presenting display with a sufficiently high contrast ratio even when observed in a direction tilted with respect to the normal to the display plane (the direction is defined by the viewing angle). In other words, widening of the viewing angle of a liquid crystal display device is desired.
To improve the contrast ratio of display presented by a liquid crystal display device, it is important to suppress light leakage in the black display state. In this aspect, a liquid crystal display device presenting black display when the liquid crystal layer thereof takes on a roughly vertically aligned state is advantageous. Examples of such a liquid crystal display device include a normally-white mode TN type liquid crystal display device and a normally-black mode vertically aligned type liquid crystal display device. These types of liquid crystal display devices present black display using a roughly vertically aligned liquid crystal layer and a pair of polarizing plates placed to face each other via the liquid crystal layer in a crossed-Nicols state. The black display presented by these liquid crystal display devices is good when observed in the direction normal to the display plane. However, when observed in a direction tilted from the normal to the display plane (hereinafter, such a direction is referred to as a “tilted viewing angle direction”), the black display degrades in quality due to occurrence of light leakage.
The light leakage in a tilted viewing angle direction occurs because (1) birefringence is generated when the liquid crystal layer in a vertically aligned state is observed in a tilted viewing angle direction and (2) the transmission axes of the pair of polarizing plates placed in the crossed-Nicols state are deviated from the mutual orthogonal relationship (the angle formed by the transmission axes exceeds 90°) when observed in a tilted viewing angle direction.
For example, Japanese Laid-Open Patent Publication No. 2000-39610 discloses that in a normally-black vertically aligned type liquid crystal display device, light leakage in a tilted viewing angle direction can be suppressed by (1) compensating retardation of a liquid crystal layer in the black display state with an optical sheet having negative uniaxial anisotropy and (2) providing an optical sheet having biaxial anisotropy that can be equivalent of a λ/2 plate (half-wave plate) having a slower axis parallel or perpendicular to the transmission axis (also called the polarization axis) of a polarizing plate.
However, when a uniaxial anisotropic optical sheet and a biaxial anisotropic optical sheet are designed according to the disclosure of the above publication, it is found that the selection range allowed for the biaxial anisotropic optical sheet capable of attaining good black display in a tilted viewing angle direction is so narrow that industrial use of this optical sheet is difficult.
Moreover, when the biaxial anisotropic optical sheet having in-plane retardation of 190 nm or more described in the above publication is used, it is found that the margin allowed for alignment between the slower axis of the biaxial anisotropic optical sheet and the transmission axis of the polarizing plate is small. Therefore, slight misalignment will cause light leakage even in the direction normal to the display plane, and this will degrade the front contrast ratio.
The aforementioned publication also describes that it is preferred to use a biaxial anisotropic optical sheet of which Nz (Nz=(nx−nz)/(nx−ny) where nx, ny and nz are the refractive indices of the biaxial anisotropic optical sheet in the slower axis direction, in the faster axis direction and in the thickness direction, respectively) is in the range of 0.28 to 0.67 (see FIG. 4). It is however difficult to industrially manufacture a biaxial anisotropic optical sheet having Nz of less than 1.0 (that is, nx>nz>ny).